Field of the Invention
The invention is generally related to an apparatus and method for measuring structural loading response in risers and pipelines in deepwater oil and gas applications, and is specifically directed to instrumentation and methods for measuring the structural response of the riser by utilizing instrumentation in direct communication with the strakes and fairings attached to the riser.
Discussion of the Prior Art
Strakes and fairings are typically installed on risers in deepwater applications to help suppress vortex induced vibration that occurs from ocean current motion. The vibration can be significant enough to cause rapid fatigue that can lead to early failure of the riser. This phenomenon affects a vast array of structures, particularly in offshore locations, and is due primarily to ocean current interacting with the riser. Other subsea structures may be affected to current loading, such as tendon legs and mooring lines.
Vortex induced vibration (VIV) of offshore structure originates when fluid (either gas as in wind, or liquid as in seawater, for example) passes a bluff body and causes low pressure eddys or vortices to form downstream of the body. The vortices are shed periodically, at frequencies that are fluid velocity dependent. Typically, vortex shedding induces loading on the body normal to the direction of current flow. For example, a riser is generally vertical and the VIV is caused by generally horizontal sea currents. This will create amplitudes of oscillation, often severe enough to damage the riser components. The oscillation is typically induced when the interaction between the flow and the structure motion causes lock-in, or becomes generally continuous and repetitive. This cyclic motion creating tension and changing curvature initiates fatigue in the structure, at time causing failure.
Marine risers are among the most common structures to be subject to VIV in offshore applications. Specifically, the design of the catenary riser, the top tensioned risers, flexibles and the drilling risers are affected by VIV. Deep water risers, typically steel catenary risers, are particularly susceptible to VIV. These risers tend to have high natural cycles or modes. High currents can excite high modes in these structures. Typically, the higher the mode number results in larger curvature and greater damage to the structure can result.
In the analysis of the effect of VIV on deep water risers it is necessary to consider the effect of both long term and short term extreme event, for example, distinguishing between a strong continuous current and a storm induced surge. Prior art analysis apparatus and methods are deficient because of the inability to obtain useful, timely data along the length of the riser and along the full water column.
Well known programs for predicting VIV damage are SHEAR7, VIVA and VIVANA. These empirical tools are based on modal calculations. While the programs are sufficient for managing the empirical data, it has remained difficult to obtain accurate, timely data relating to the actual cyclic activity of the riser along the water column. The lack of measured data, and the reliance on modeling, has made the calculations estimates, at best. Because of the uncertainty in the data, operators generally require safety factors of between 10 and 20 in the design process.
VIV suppression devices have been developed to reduce the vortices and/or disrupt their formation. Two of the most common suppression elements are helical strakes and fairings. A helical strake consists of a number of fins wound as a helix around the periphery of the riser. The helical strakes shed the vortices into finite cells and length, both shortening and weakening the vortices. Fairings are aerofoil-shaped structures that streamline flow and reduce the VIV by weakening vortices that are shed at that location.
In some cases monitoring systems have been installed on a riser prior to its deployment in deepwater. Such systems typically consist of fiber optic sensors or in some cases accelerometers have been attached in an effort to determine fatigue and structural response.
Fiber optic sensors have provided the most accurate, reliable and efficient attachment methods to manage data. Attachment methods have involved independent sensor stations located at preselected segments along the riser. This novel system prescribes a method that places the sensors directly within the suppression device (for example, a strake or fairing).
It is desirable to obtain accurate, near real-time data relating to the effect of VIV on risers and similar structures in a timely and cost-effective manner in order to promote more efficient and effective riser design, thereby both increasing the safety and also perfecting the design of such structures. By incorporating the sensors within the suppression device, the desired measurements are obtained in a simple and efficient manner to incorporate the valuable data in the monitoring system.